1. Field of the Invention
This invention relates to a process of directly reducing iron oxide-containing materials by a treatment with solid carbonaceous reducing agent below the melting point of the charge in a rotary kiln, which comprises preheating the iron oxide-containing material and the carbonaceous reducing agent in a multiple-hearth furnace to at at least partial coking or an at least partial low-temperature carbonization.
2. Discussion of the Prior Art
The use of solid reducing agents, which contain a very high proportion of volatile constituents, such as lignites or brown coals, in a rotary kiln rise to an evolution of gas at a high rate. This gas rate will be increased further if moisture is contained in the reducing agent or in the iron oxide-containing material. The high gas rate results in a high velocity of flow of the furnace atmosphere. Because in a rotary kiln operated with countercurrent flow of the furnace atmosphere and the charge, the gas has the highest velocity at the charging end, the exhaust gas entrains fine-grained solids at a considerable rate, particularly if the charge contains fine-grained ores and a reducing agent having a relatively high proportion of fines. By these entrained solids, an afterburning of the combustible volatile constituents still contained in the exhaust gas is rendered more difficult. Such afterburning is necessary for economic reasons in case combustible volatile constituents become available at a rate which greatly exceed the rate required to supply the heat demand of the rotary kiln. Additionally, the rotary kiln is a relatively poor heat transfer system so that the heating-up zone is relatively long. This is true also where pellets or coarse ores are charged into the rotary kiln.
It is known from Offenlegungsschrift No. 2,741,216 to preheat a mixture of fine-grained iron ores and solid reducing agent in a multiple-hearth furnace to a temperature at which an at least partial coking or an at least partial combustion of the reducing agent is effected before the mixture is charged to the rotary kiln. In that multiple-hearth furnace, the required heat is supplied by the gases produced by low-temperature carbonization. That preheating step may result in local temperature rises above the permissible upper limit and in sintering. Additionally, it is not possible to use caking coals, and the ore is partly reduced so that crusts may be formed. If coarse ores or pellets are charged, their partial reduction results in their disintegration.
It is an object of the invention, therefore, to avoid said disadvantages and to provide a process which, in turn provides an economical preheating of iron oxide-containing materials with the aid of combustible volatile constituents of solid reducing agents.